What do you get when you look at social media as a source of information about people? This topic usually goes off into a discussion of influence, a result of thinking of social media as media. What if, instead of influencers, you think of the people who participate in social media as individuals?
All you need to start is an indentifier—a name, email address, Twitter handle—and you start connecting dots. When people include, for example, a Twitter handle in a LinkedIn profile, you can have real name, location, employment, schools… Maybe links to more networks to continue the process, too.
It's not a question of probabilities. When people create links between their various network profiles, it's a clear statement that the accounts belong to the same person.
Why build when you can buy?
This is another niche for startups, of course. Several of which are working to reconcile public profiles across multiple social networks and using that information to create information-rich individual profiles. To make it even more useful, most of these companies offer APIs for integrating their profile data into other systems.